The usual wood screw comprises a metallic rod or wire which has one end deformed into an enlarged head, generally by upsetting in a suitable swage, and has an opposite extremity formed with helical threads of a rather coarse pitch centered on the rod axis. The turns may extend over the full length of the rod up to its head but in many instances are separated therefrom by an unthreaded, i.e. cylindrical, rod portion designed to support a hinge, a bracket or some other attachment to be fastened to the substrate. Conventionally, the thread is formed by milling or chasing a helical groove into the rod extremity remote from the head so that the outer diameter of its turns equals that of the unthreaded portion or shank; this is considered desirable in order to eliminate any significant play between that shank and the surrounding rom of a hole in the attachment to be fastened which of course must be large enough to give passage to the thread. The cutting of the thread may be performed on a blank on which the head may have already been swaged, this head being usually provided with a transverse slit, a hexagonal recess or some other formation designed to be engaged by a tool driving the screw into the substrate.
The machining of such a thread causes a loss of material of more than 20% in many instances. In order to avoid this waste, is is known to reduce the diameter of the rod portion to be threaded by passing it through a suitable die, of the type used in wire drawing, and to subject that rod portion thereafter to a rolling process which forms the thread without chip removal. The thinning of this rod portion prior to deformation by the roller has the purpose of preventing the resulting thread turns, produced by displacement of metal from the rolled groove, from significantly exceeding the diameter of the unthreaded part of the rod. As with the aforedescribed machining operation, the end of the rod opposite the head must be shaped into a point to facilitate the initial driving of the screw into the workpiece. Even so, it is frequently necessary to predrill the substrate to form therein a bore having the diameter of the thread core, an inconvenience which also tends to weaken the hold of the substrate on the scew.
While the rolling of the thread saves metal, the initial thinning of the rod--which the art appears to have heretofore considered indispensable--still constitutes a costly and somewhat cumbersome operation. On the other hand, woods screws with rolled-on threads have greater mechanical strength than those of like dimensions produced by machining.